Production of hydrocarbons from subsurface reservoirs



J. M. BARRON Det. 14, 1958 2 Sheets-Sheet 1 Filed July 2o, 1954 @3% mugh Smm Q www M OO QN Y QM QW k k. m.

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Get. M, 1958 J. M. BARRON 2,856,000

l PRODUCTION OF HYDROCARBONS FROM SUBSURFACE RESERVOIRS Filed July 20,1954 2.Sheets-Sheet 2 2,856,0ilil PRODUCTION F HYDRCARBONS FRMSUBSURFACE RESERVlRS.

Joseph M. Barron,` Port Arthur, rlex., assigner to Texaco DevelopmentCorporation, New York, N. Y., a corporation of Delaware Application July2l), 1954, Serial No. 444,610

1 Claim. (Cl. loe-9) This invention relates to the production ofhydrocarbons from vsubsurface reservoirs. More particularly,thisinvention relates to a method forl increasing the overall;production of normally liquid hydrocarbonsffrom subsurface reservoirs.

A gas` condensate reservoir is a subsurface reservoir iu which the'bulkof the hydrocarbons exists as a high pressure gas subject tocondensation upon isothermal pressure decline. Accordingly, a retrogradeenriched high pressure gas comprises a single phase admixture ofnormally gaseous hydrocarbons, such as methane and ethane, together withenriching constituents comprising normally liquid hydrocarbons. In aretrograde enriched gas the amount of the enriching normally liquidhydrocarbons contained therein is far in excess of the amount whichcould be held by the normally gaseous hydrocarbon constituents thereofas a gas at normal atmospheric pressure. Such a subsurface high pressurereservoir is produced by withdrawing the retrograde enriched gas to thesurface, subjecting it to a reduction in pressure and permitting theconstituents thereof to separate into a gaseous natural gas fraction anda .liquid hydrocarbon condensate fraction.

It is an object to provide an improved method and apparatus forproducing hydrocarbons from a plurality of subsurface reservoirs.

It is another object of this invention to provide an improved method forAproducing hydrocarbons from subsurface reservoirs wherein at least oneof the reservoirs is originally at a substantially higher pressure thanat least one other of said reservoirs.

It is still another object of this invention to provide an improvedmethod for producing hydrocarbons from a -high pressure reservoirunderlying a relatively low pressure reservoir.

Stilljanother object of this invention is to provide an improvedk methodfor producing hydrocarbons from a highV4 pressure reservoir containingnormally liquid hydrocarbons in retrograde admixture with normallygaseoushydrocarbons wherein said high pressure reservoir is-'associatedwith an overlying relatively low pressure reservoir which containsnormally liquid hydrocarbons therein as a separate liquid phase.

It4 is still another object of this invention to provide a method andapparatus for recovering normally liquid hydrocarbons from a relativelyhigh pressure reservoir without employing extensive separating andfractionating equipment on the surface.

At least one of the objects of this invention will be accomplished in atleast one embodiment of this invention.

Theseand-other objects of this invention and how they are accomplishedwill become yapparent with reference to the accompanying disclosure anddrawing where- Fig. g 1 schematically illustratesl a vertical sectiongthrough geological subsurface formationswhich include atent pressure.

lTice a relatively high pressure hydrocarbon producing reservoir and arelatively low pressure hydrocarbon producing reservoir;

Fig. 2 illustrates an apparatus which might be employed in the practiceof my invention for controlling the ow of a retrograde enriched fluidfrom a high pressure formation to a low pressure formation;

Fig. 2a is a partial cross sectional View of the apparatus of Fig. 2taken along the line A-A; and

Fig. 3 graphically illustrates some of the advantages to be obtained inthe practice of my invention.

In accordance with my invention the contents of a relatively highpressure gas condensate reservoir after having been at least partiallystripped of some of its retrograde enriching hydrocarbons are dischargeddirectly into a separate, relatively low pressure hydrocarbon reservoir.The low pressure reservoir is then produced by means of a producing wellextending thereinto at a point spaced from the point of introduction orinjection of the high pressure gas obtained from the high pressurereservoir. As a result of the above-indicated operations, the contentsof the high pressure reservoir and the low pressure reservoir areproduced from the relatively low pressure reservoir through theproducing well extending thereinto. It is contemplated in the practiceof my invention to direct substantially all of the remaining gasiformhydrocarbons from a relatively high pressure condensate reservoir intoand through a relatively low pressure reservoir. Advantageously therelatively low pressure reservoir contains gas relatively rich innormally liquid hydrocarbons with respect to the high pressure gasinjected thereinto.

In accordance with the present invention, it has been found that therelatively lean high pressure gas entering the relatively low pressureand relatively rich condensate reservoir not only increases the pressureof the low pressure reservoir, but also the lean high pressure gasbecomes enriched by taking up by retrograde vaporization the normallyliquid phase hydrocarbons present in the lower pressure condensatereservoir. The enrichment which takes place raises amount of normallyliquid hydrocarbons contained in the resulting retrograde enriched phaseto a value corresponding to the enrichment characteristic of therelatively rich condensate reservoir at the pressure newly prevailing.Therefore, the amount of Vnormally liquid hydrocarbons or condensate soproduced from the two reservoirs is substantially greater than theamount which could be produced if the two reservoirs were separatelyproduced.

For example, the high pressure relatively lean retrograde fluid passinginto the relatively low pressure condensate reservoir picks up orexchanges hydrocarbons until it reaches an equilibrium compositioncorresponding to that of the conditions prevailing in the low pressurereservoir. Since opening up or connecting the two reservoirs results insubstantially increasing the pressure of the relatively low pressurereservoir from which production is effected, the total retrogradeenrichment of the produced fluid is increased. This follows from thefact that, in general, the amount of normally liquid condensatecontained in the retrograde enriched gas delivered by a given reservoirincreases with the reservoir As the pressure decreases the amount ofnormally liquid fractions recovered as condensate decreases;` stated inmore practical terms, the number of barrels of stock tank liquidrecovered per MMCF of producedl As a result, therefore the normallyliquid phase hydrocarbons which may be lefts in the relatively richcondensate reservoir by virtueof normal pressure decline, are recoveredby producing a relatively high pressure and relatively lean uid derivedfromfa high pressure reservoir through the relatively rich condensatereservoir.

The benefits of this operation are particularly enhanced by virtue ofthe fact that a typical condensate reservoir frequently contains, inaddition to the retrograde enriched phase which forms the bulk thereof.a separate liquid phase of hydrocarbons which may be referred to asinterstitial liquid. While the interstitial liquid may be too small inamount to permit its separate recovery, nevertheless it is obviouslyavailable for enriching the injected high pressure lean gas. It diffusesinto the high pressure gas injected from the relatively high pressureformation and is finally recovered as condensate. Accordingly, inaddition to recovery of liquid which would normally be condensed andlost in the relatively rich low pressure condensate reservoir due tonormal pressure decline in the course Vof production, the presentinvention provides for recovery of the interstitial liquid phasehydrocarbons which otherwise would form a part of the irrecoverable oil,i. e. the oil remaining in the formation after completion of normalproduction.

In theoretical explanation of the foregoing it is believed that theincreased enrichment of the relatively high pressure lean gas whichoccurs in the relatively lowpressure reservoir results from an actualequilibrium eX- change of constituents thereof. In the low pressurereservoir there is a readjustment of the injected high pressure gas asabove indicated, which results in an enrichment corresponding to that ofthe relatively rich low pressure reservoir. In any event, theequilibrium composition of the resulting retrograde enriched gas phaseadjusts itself to that characteristic of the relatively rich reservoir,and this equilibrium adjustment significantly occurs at a relativelyrapid rate and is completed during a production operation in accordancewith this invention.

This invention is also applicable to the production of liquid oilreservoirs. Injection of a relatively high pressure lean gas into theliquid hydrocarbon reservoir drives the liquid toward the productionwell, dissolves in the liquid oil to swell its volume which increasesthe volumetric displacement eiciency. Moreover, some of the reservoirliquid diffuses into the resulting retrograde enriched gas phase of thecondensate reservoir to effect increased enrichment thereof, as aboveindicated, so that additional amounts of condensate are produced withthe recovered enriched gas.

Moreover, any enriching fractions contained in the injected highpressure sweep gas, to the extent they are absorbed in the liquid oil,swell the volume of the formation liquid oil, thereby facilitating itsflow to the production wells. This follows from the fact that theenriching fractions contained in the high pressure injected gas aretypically composed of relatively light liquid fractions, for example inthe gasoline boiling range, which tend to lower the overall density andviscosity of the formation liquid oil. As a result the crude flows morefreely to the production well.

In connection with the preferred embodiment of thc present invention,reference is made to Fig. 1 of the drawing which illustrates in verticalsection the ground surface and two vertically spaced condensatereservoirs 12 and 14. The two reservoirs are separate and as is typical,the lower reservoir 14 has a substantially higher original reservoirpressure than the upper reservoir 12.

For example, the overlying relatively low pressure and relatively richreservoir 12 in its original state contained about 500,000 MMCFcondensate (i. e., retrograde enriched gas phase) at 6700 p. s. i.andoriginally delivered 68 barrels of stock tank liquid/MMCF of gas. Thelower relatively high pressure condensate reservoir 14 4 originallycontained 430,000 MMCF gas at 7600 p. s. i.. and produced 37.4 barrelsof stock tank liquid/MMCF.

As indicated in Fig. 1, lthe high and low pressure reservoirs areproduced in accordance with this invention as follows. A casing 11extends from the surface downward into the hydrocarbon producingformations and is perforated within the low pressure formation 12 andhigh pressure formation 14. Positioned within casing 11 is liquidconduit 13 which extends downwardly from the surface to a point beneaththe bottom of the low pressure formation 12. A third conduit orso-called flow tubing 15 is positioned within the liquid conduit 13 andextends downward from the surface to a point above the upper surface ofthe high pressure formation 14. A flow control means or valve, shown ingreater detail in Figs. 2 and 2a of the drawing, for controlling the owof the high pressure fluid from high pressure formation 14 to lowpressure formation 12 is provided at about the lower end of tubing 15. Acolumn of contact material 17, such as Raschig rings, Berl saddles orsimilar gas-liquid contact promoting material is provided in the annulusbetween casing 11 and ow tubing 15 intermediate the lower end of owtubing 15 and the lower end of liquid conduit 13. As indicated the massof contact material 17 is also located within casing 11 intermediate thebottom of the low pressure formation 12 and the top of the high pressureformation 14. A pump 18 in uid communication with liquid conduit 13 viaconduit 19 is provided for pumping absorber oil, such as a strippedcrude oil having desired absorbing properties into conduit 13. The flowof produced rich oil and gas which flowing upwardly within ow tubing 15is in part controlled by a valve 20 at the upper end of tubing 15. Ahandle or control means 21 is connected to the upper end of tubing 15for turning the same thereby operating the ow control means or valvepositioned at the lower end of ow tubing 15 and located intermediate theformations 12 and 14.

Referring now to Fig. 2 of the drawing which shows in greater detail avalve arrangement for controlling the flow of high pressure fluid fromreservoir 14 through the mass of contact material 17 and for controllingthe flow of produced oil and gas upwardly within tubing 15, there isshown a support means 22 which is fixed to the inside of casing 11 bythreaded engagement or by Welding or by some sort of locking arrangementso as to prevent a rotation thereof with respect to casing 11 when theow tubing 15 is turned. As indicated in Fig. 2, a support means 22serves not only as a support for perforated plate 23 which in turnsupports the mass of contact material 17 but also serves as the valvebody of the valve arrangement. Support means 22 as illustrated is in theshape of a truncated cone provided with a downwardly extending ange 22awhich is closed at its lower end 25 by means of a plate or similarclosing member to form a liquid trap or reservoir. Openings 27 areprovided in the walls of cylindrical flange 24 for the passage of highpressure fluid therethrough. Arcuate lugs 27a are fixed to the ow tubing15 and are adapted to cooperate with openings 27 to control the amountof tluid passing therethrough as tubing 15 is rotated. Fig. 2a showsmore clearly the arrangement for controlling the ow of high pressurefluid through openings 27. As indicated in Figs. 2 and 2a, tubing 15 isprovided with arcuate lugs 27a which act to close off openings 27 andthereby control fluid flow therethrough.

The assembly illustrated in Figs. 1, 2 and 2a may be installed in thefollowing manner. A string of ten or twelve inch casing 11 is runthrough both the high pressure 14 and the low pressure 12 formations andperforated opposite each formation, the casing having been provided withsupport means 22 positioned just above the top of the high pressureformation. Upon the support means 22 is set the perforated plate orcontrol material support member 23. As indicated hereinabove, a lockingarrangement is provided between the support means or" valve body 22 andthe casing, since thevalve body must remain in a fixed position whentheow tubing 1,5 is turned. Contact material A17 isplaced within theannulus between the casing 11 and flow tubing 15, intermediate the high`and low pressure formations, preferablyin the following manner. Asectionv .of Wire screen. 28, the length of a joint of tubing,.andhaving a Ydiameter slightly less than the inside diameter vof the casing11 isset on the perforated plate 23 and filled with 1/2 inch `or 3A inchBerl saddles. As each length .of tubing, is lowered within thecasingstringl another section of screen is added and ille'd with packing. Thisoperation is continueduntil the desiredamount of packing has beenprovided. Additional tubing is added until vthe valve assembly rests onthe ,support means 22 just above the top of the high pressure` formation14., The lpackinglfl and tubing 15 rest onthe supportmeans 22. The,liquid conduit, 13 for absorber -oil is run into casing 11 to a pointjust above the top of the packing 17which isbelow the bottom of thelowpressure formation 12. Surface connections. are then, provided asindicated in Fig. l so that'absorber oil can be introduced into theannulus between the ow tubing 15 and liquid conduit 13 to thel packing.During operation` a rich oil accumulates in the trap defined by iiange22a and closing plate 25 below the openings'27 of the flow controlarrangement and is swept to thel surface through the valve provided atthe surface at the upper end of the flow tubing 15. The flowcontrolvalve, i. e. position of lugs 27a with respectto openings 27, isadjusted to maintain the flow of the high pressure gas or fluid to theannular column of contact material V17 below the flooding point. Inactual practice the desired flow control valve settings willbedetermined by trial and error. It is contemplated that anabsorber ofthe type illustratedv in Fig. 1, such as` an absorber having a diameterof twelve inches and a length of several hundred feet (200400), suppliedwith a proper type of absorber oil, such as normally liquid hydrocarbonsor mixture thereof having an average molecularfweight in the range150-250, preferably about 200, would effectively remove the C3 andC4hydrocarbon components and a substantial portion of the heavier normallyliquid hydrocarbons from the high pressure gas fluid emanating! fromthehigh pressure formation 14 to produce a relatively lean highpressure` gas for injection into and passage through lthe relatively lowpressure formation 12.

Fig. 3 represents diagrammatically the. contents of the two reservoirsas a function of pressure.` The curve AB represents the instantaneouscontent of liquid hydrocarbon production from reservoir 12 per MMCF ofproduced gas between the pressures 6700 and 5200 p. s. i. Curve CD is asimilar production curve from reservoir 14.

The reservoir 12 contains 1690 MMCF and the reservoir 14 contains 1200MMCF of hydrocarbon pore space. On the above basis the total quantity ofliquid produced by direct production of reservoir 14 during the 7600-5200 p. s. i. pressure decline is represented by the area under thecurve CD. This amounts to approximately 3,940,000 barrels. Likewise thearea under curve AB represents the quantity of liquid recoverable byseparate and independent production of reservoir 12. The area abovecurve AB represents the amount of liquid condensate which remains in theinterstices of the porous reservoir 12 as the result of condensationduring the pressure decline from 6700 to 5200 p. s. i. as reservoir 12is produced.

In accordance with the present invention, after the production of thereservoir 12 to a pressure of 5200 p. s. i., connection is made betweenthe two reservoirs 12 and 14 as indicated in Fig. l. passes upwardlywithin the packed annulus between casing 11 and tubing 15counter-currently in contact with absorber oil and is stripped of asubstantially amount of its retrograde vaporized normally liquidhydrocarbons ,to yield a relatively lean high pressure gas for injectionFluid from formation 14 f into formation 12, the stripped normallyliquid hydrof carbons together with the absorber'oil beingrecovered fromflow tubing 15 at the surface via-valve 20.`

As indicated, see Fig. l, reservoir 12 in turn` is produced by means ofa spaced well 30 extending lfrom a surface location into the formation12.. Flow tubing 31 of well 30 is connected to theI well head 32 whichdirects the produced hydrocarbon to stock tank, separators I.and anyother desired recovery means not shown.,

Further explanatory of my invention, as the reservoir 12 is producedthrough well 30 its pressure slowly declines to about 5200 p. s. i., atwhich time about 47 bare rels of stock tank liquid are produced per MMCFof produced gas, see curve AB of Fig. 3. At thistime the reservoir 14 isadvantageously interconnected to reser- Voir 1T. by perforating casing11 in both reservoirs. The resultant pressure after equalization isapproximately 6200 p. s. i. in both reservoirs. At thispressureenrichment of the injected relatively lean high pressureretrograde gas phase from formation 14, indicatedby curve AB, takesplace by retrograde vaporization of the liquid hydrocarbons and/orpreviously condensed liquidhydrocarbons in the reservoir 12. Asdisclosed this enrichment results in delivery from well 30 of acondensate production yielding 56 barrels of stock tank liquid per MMCFof gas. Therefore, production isresumed through well 30 at the increasedyield of 56 barrels of stocktank liquid/MMCF of gas which contrasts withthe previous production of 47 barrels from formation12.

As production continues fromboth formations 12 and 14 via well 30 thepressure gradually declines in both formations and the yield of stocktank` hydrocarbons again follows the pattern represented by curve AB ofFig. 3the shaded area under curve AB between 6200 and 5200 p. s. i.,representingtotal production attributable to the upper reservoir 12between these pressure limits after interconnection with reservoir 14 inthe manner indicated. On the basis of the pore volume .of reservoir 12this production amounts to 4,390,000` barrels of liquid oil. However,due to the interconnection of reservoir 14 the rate of pressure declinelis correspondingly less over the same pressure differential, and thereis additional production attributable to the increased pore space of thereservoir 14. The total increase in reservoir volume represented bycombining reservoir 14 with reservoir 12 amounts `to or 1.71; thereforethe total recovery is equal to 4,390,000 X 1.71 or 7,550,000 barrels.

This compares very favorably with only 3,940,000 barrels which would beproduced during the pressure decline of the reservoir 14 between itsoriginal pressure of 7600 p. s. i. and the pressure of 5200 p. s. i. byordinary direct production and representing an increased production of3,6l0,000 barrels of valuable liquid hydrocarbons.

While the striking results of the foregoing example are based, in part,upon the presence of original interstitial hydrocarbon liquid inreservoir 12, it is to be noted that in any event the originalproduction of reservoir 12 from its initial pressure of 6700 down to5200 p. s. i., as previously indicated necessarily resulted inretrograde condensation to an extent signified by the shaded area abovecurve AB of Fig. 3. The actual amount of liquid oil deposited byretrograde condensation during this pressure decline is equal to1,920,000 barrels. Accordingly, therefore, even in the absence ofinterstitial liquid, the production of this very material quantity ofadditional liquid hydrocarbon manifestly represents a valuableadvantage.

This increased production, in either case, is particularly advantageousfrom the standpoint that the increased production is readily predictableby a skilled reservoir engineer on the basis of ordinary engineeringdata regarding the reservoir. The present invention is particularlyapplicable to deep elds when the costs of cycling or other conventionalmethods of secondary recovery employing extensive surface facilitatesare economically prohibitive.

As previously indicated, similar advantages follow from the injection ofthe contents of the relatively high pressure condensate reservoir to arelatively low pressure liquid oil reservoir. This is particularly truein the case of a reservoir containing oil which is undersaturated withgas at reservoir pressure. swelling of the oil volume and the increaseof fluidity, due to contact with the enriched condensate phase,materially promote ow to the well bore.` Recovery is further facilitatedby the fact that the resulting retrograde enriched condensate phase,having properties as to viscosity and density more closely approachingthat of the liquid oil, acts with improved volumetric displacementeiciency. In any event, the total amount of liquid hydrocarbons producedby the methods of the present invention is materially increased.

In one modification of the present invention the relatively lean (withrespect to retrograde vaporized normally liquid hydrocarbons) contentsof a high pressure condensate reservoir may be injected successivelyinto a number of relatively low pressure reservoirs. For example, thecontents of a high pressure relatively lean condensate reservoir may rstpass through to a lower pressure relatively rich condensate reservoir,as in the above example, and thereafter pass through into a liquid oilreservoir. Production is then carried out from the liquid oil reservoirat a point spaced from the point of which the resulting condensate gasis injected thereinto. Therefore, the first reservoir discharges throughthe second condensate reservoir and from the second reservoir throughthe liquid oil reservoir, all of the several reservoirs being producedby a well or wells extending exclusively into the oil reservoir. In thismanner each of the sands is produced under conditions of maximum liquidhydrocarbon recovery.

This application is related to the copending, co-assigned patentapplication, Serial No. 388,113, now Patent No. 2,736,381, led October26, 1953, in the name of Joseph C. Allen, which discloses producing aplurality of subsurface reservoirs wherein retrogradeuid from anunderlying high pressure reservoir is without any intervening treatmentinjected directly into an overlying relatively low pressure condensatereservoir.

In such a case the resulting Obviously many modifications andvariationsmay be made by those skilled in the art without departing fromthe. spirit and scope of this invention.

A method of producing subsurface gas condensate reservoirs wherein arelatively rich, relatively low pressure gas condensate reservoiroverlies a relatively rich, relatively high pressure gas condensatereservoir which comprises placing said gas condensate reservoirs indirect uid communication by means of a rst well bore penetrating saidlow pressure and high pressure condensate reservoirs, providing anabsorption zone within said first well bore intermediate said condensatereservoirs and interposed in the tlow path of direct uid communicationtherebetween, owing the contents of said high pressure gas condensatereservoir directly into said absorption zone,introducing viasaid rstwell bore alliquid hydrocarbon oil having an average molecular weight inthe range -250 as an absorption medium into said absorption zone intodirect contact with the aforesaid contents of said high pressure gascondensate reservoir introduced into said absorption zone to remove asub- Ystantial amount of normally liquid hydrocarbons as well as asubstantial amount of C3 and C4 hydrocarbons from the high pressuregaseous materials introduced into said absorption zone from said highpressure gas condensate reservoir, removing the resulting enrichedliquid hydrocarbon oil from said absorption zone, directly via said rstWell bore introducing the resulting relatively lean high pressuregasiform eluent from said absorption zone into said relatively lowpressure condensate reservoir and producing said low pressure condensatereservoir via a second well bore spaced from said rst well bore andpenetrating only into said low pressure reservoir whereby both said highpressure and said low pressure condensate reservoirs are produced viasaid second well bore.

References Cited in the file of this patent UNITED STATES PATENTS1,433,956 Knox Oct. 31, 1922 2,185,577 Voorhies et al. Jan. 2, 19402,238,701 McCollum Apr. 15, 1941 2,277,380 Yancey Mar. 24, 19422,357,703 Teichmann Sept. 5, 1944 2,368,428 Saurenman Jan. 30, 19452,423,156 Reid July 1, 1947 2,736,381 Allen Feb. 28, 1956

